The present invention is in the field of glass-plastic laminates, and particularly relates to the production of curved glass-plastic laminated lenses exhibiting good photochromic properties.
Photochromic glasses are glasses which darken on exposure to visible or ultraviolet light and return to the clear state in the absence of such light. Examples of photochromic glasses are shown in U.S. Pat. No. 3,208,860 to Armistead et at. Photochromic glasses have been used to fabricate ophthalmic and sunglass lenses which provide automatic darkening under bright lighting conditions.
Plastic ophthalmic lenses have an advantage over glass lenses in that they are relatively light in weight. However,to date no plastic material exhibiting reversible photochromic behavior which is free from fatigue (i.e., gradual loss of photochromic properties upon repeated use) has been developed.
It has previously been proposed to combine the desirable properties of photochromic glasses and plastics in glass-plastic laminated articles wherein photochromic glass would impart the desired fatigue-free photochromic behavior. Hence, published German Pat. application No. 1,284,588 suggests a variety of ways in which the desirable properties of photochromic glasses and plastics could be combined. A technique for bonding glass to high shrinkage allyl diglycol carbonate plastics is described in the copending commonly assigned application of A.A. Spycher, Ser. No. 27,231, filed Apr. 5, 1979 now U.S. Pat. No. 4,227,950, a continuation of Ser. No. 848,942, filed Nov. 4, 1977 now U.S. Pat. No. 4,168,068 . Nevertheless, there is at present no commercially successful product wherein a satisfactory combination of photochromic glass and plastic has been achieved.
The absence of any commercially successful product in this field can be attributed to the difficulty of providing a laminate exhibiting the essential properties of optical clarity, acceptable photochromic response, and, most importantly, the mechanical durability necessary to enable the composite to retain its physical integrity under the adverse conditions of use. Concerning the problem of mechanical durability, the coefficient of expansion of most glasses falls within the range below about 9.times.10.sup.-6 /.degree. C., while that of commercially available allyl diglycol cargbonate resins presently used for plastic ophthalmic lenses is on the order of 100.times.10.sup.-6 /.degree. C. This expansion mismatch readily causes stresses which induce glass layer fracture or glass-plastic delamination in a glass-plastic composite article even as it is cooled to room temperature after processing. Because of this mismatch, the production of a laminated lens which can withstand reasonable temperature excursions (e.g., from about -40 to about 100.degree. C.) in use is extremely difficult.